The present invention relates to data communications equipment, e.g., modems. In particular, this invention relates to the transmission of both voice and data signals over the same communications facility at the same time.
The co-pending, commonly assigned, U.S. Patent application of Gordon Bremer and Kenneth D. Ko entitled "Simultaneous Analog and Digital Communication," Ser. No. 08/076,505, filed on Jun. 14, 1993, discloses a simultaneous voice and data communication system in which a voice signal is added to a data signal for transmission over a communications channel to a receiving modem. In particular, the data signal to be transmitted is represented by a sequence of data symbols, where each data symbol is associated with a particular N-dimensional signal point value take from a signal space. Similarly, the voice signal is processed so that it is mapped into the N-dimensional signal space to provide a voice signal point. This voice signal point defines the magnitude and angle of a voice signal vector about the origin of the signal point constellation. The data symbol and the voice signal vector are then added together to select a resultant N-dimensional signal point.
Since this resultant N-dimensional signal point is a function of both the particular data symbol and the magnitude and phase of the voice signal vector, the signal space is essentially divided into a number of square non-overlapping regions, each region comprising a number of signal point values with one signal point value being associated with a data symbol. Consequently, any signal point value within a square region may be validly selected for transmission to a receiving modem. Since each signal point region is non-overlapping, the size of each region is fixed. This results in limiting the magnitude, or dynamic range, of the voice signal vector so that the resultant signal point always lies within a region. The dynamic range of the voice signal vector is directly related to the transmission quality of the voice signal.
Typically the overall size of the signal space is constrained by power limitations. As a result, the modem's data rate and quality of voice transmission are interrelated. For example, for higher voice quality, the signal space is divided into a smaller number of larger size square regions. Each square region is associated with a data symbol and the data rate is directly proportional to the number of data symbols available for transmission. Since there are fewer, but larger, square regions, the dynamic range of the voice signal, and therefore its quality of transmission, increases at the expense of a lower data rate, which is the result of fewer available data symbols. However for lower voice quality, the signal space is divided into a larger number of smaller size square regions. Now since there are more, but smaller, square regions, the dynamic range of the voice signal, and therefore its quality of transmission, decreases with the benefit of a higher data rate. Consequently, any attempt to increase the data rate results in a deterioration in the quality of voice transmission.